The reduction in size of electrical assemblies in the past 25 years has resulted from more-or-less distinct stages of technological advancement. The first stage was associated with the replacement of vacuum tubes by various solid state devices, such as transistors. Another stage accompanied fabrication of such devices on a single, discrete piece of material, as a chip or wafer. Later stages accompanied the manufacture of a complete assembly of such devices, along with inactive components as resistors and capacitors, on a single chip, resulting in an integrated circuit which first were merely large scale integration, (LSI), then very large scale integration (VSLI).
Accompanying the evolution of ever decreasing size of components and of integrated circuits was a concurrent development in packaging the assembly of such devices to achieve minimum finished product size. The printed circuit board has continued to represent an effective physicl medium for architectural construction of the completed assembly. With improved methods for etching and continued development of laminates the printed circuit board has evolved from a rather thick plate with widely spaced components on one side to a strong, thin laminate with densely packed components on both sides.
Although reduction in component and package size have been independent advances they clearly have been interdependent advances, in the context that the advantages of advances in one area are fully realized only with corresponding advances in the other area. In relatively recent years the technique of printed circuit board assembly has changed to increasingly emphasize surface-mount devices, i.e., miniature, largely leadless components which are directly soldered to the surface of a printed circuit board rather then requiring holes in a substrate through which a lead from a component is soldered. One advantage of surface-mounted devices is an increased board density through elimination of holes, and through mounting of components on both sides of a printed circuit board. Another advantage is better performance, especially at high frequencies, resulting from the reduction in lead length. The process of surface mounting can be, and has been, extensively automated, leading to increased product reliability, greater production rate, and lowered assembly cost. The impetus for surface mounting has come from the desire for miniaturization, for improved quality and reliability, and from production economies.
Automation techniques for surface mounting devices on a printed circuit board utilize systems which pick off the devices from bulk, tapes, tubes or reels on which the devices are packaged, and precisely place the devices at their designated position on the printed circuit board, immobilize each of the devices to the printed circuit board via adhesive contact or solder paste, and finally solder the devices to make electrical connections on the board. This process in turn imposes some requirements on devices for optimum efficiency. One is that at least one surface, and preferably two opposing surfaces, be flat. The placement head of the automated system can more readily pick up the device from the tape or reel on which it is packaged if a flat surface is presented, since the pickup often is via suction. It is also desirable that the surface of the device contacting the board be flat so that after the device is placed on the board, but before the adhesive hardens, it will not move. Another requirement is that the surface-mount devices have flat leads, for if they have round leads there is minimal contact with bonding pads on the printed circuit board and subsequent soldering may lead to a relatively high proportion of poor electrical connections. This need to have an adequate solder area is often referred to by saving that the device needs to have a good footprint.
Many electrical components presently are packaged as surface-mount devices, including integrated circuits, chip capacitors and chip resistors, but the described limitations have imposed serious constraints on, e.g., the power range of surface-mounted resistors. To date the surface-mount resistors are generally in the 1/10 to 1/4 watt range, and it has been stated that surface-mount versions of power resistors are not really practical. R. J. Klein Wassink and H. J. Vledder, Philips Technical Rev., 40, 342-8(1982). In the context of this application a power resistor is one with a power rating of at least 1 watt. Reasons for the lack of an economically viable surface-mount version of power resistors include improper shape, excessive heat generation at the board surface, and flexing of soldered leads.
Power resistors, whether wire wound or metal film, traditionally are manufactured in cylindrical or tubular form, and in fact expensive retooling and redesigning would be necessary to manufacture them in another form, for example, one with two opposing flat surfaces. As a surface-mount device tubular power resistors would generate much heat at their point of contact with the printed circuit board, and since contact surface is minimal the local heat density would be intense. Cyclic heat generation also would cause stress flexing of the solder connection ultimately causing electrical failure at the solder joint or flexing along the lead which might result in its structural failure.
Prior attempts to address these problems led to power resistors with a rectangular metal cap soldered at each end to provide two opposing flat surfaces, and with round leads which have been flattened and bent. Although the problem of vacuum pickup is at least partially solved by such a device, as is the tendency of a tubular resistor to roll when placed on a printed circuit board, the problems of attending heat generation along a small contact surface and of flexure remain. In fact the difficulties arising from flexing are even more severe than if the lead is cylindrical, for the point of transition from a round to a flat lead is structurally quite weak, and the flex stress communicated is somewhat localized at just this transition point. Surface-mount power resistors of the aforedescribed design have notoriously high failure rates, entirely defeating the reliability of an assembly, normally one of the advantages of surface mounting.
Thus there are several problems which must be overcome in order to deliver to the marketplace a surface-mount power resistor. The invention described within is a surface-mount power resistor which has indeed cured all the deficiencies of prior art devices to afford an economically viable component. The solution to problems inherent in power resistors also is applicable to other devices, so that even though our emphasis has been, and will remain, on power resistors it is to be clearly understood that our solution is applicable to all axial lead compenents generally, including capacitors, diodes, and inductors.